Soon, Dr. Zed's abominations, the Skrakks will come flying at you. Use your vehicle's turret to take out a batch. If your ride gets destroyed, finish the rest using shotguns. Commandos will find that turrets get the job done fastest. You have to kill all 20.

There is a chance that a glitch will continue to make the quest marker appear in the location where the Skrakks were. Ignore it and head back to the Fast Travel Station.

Head to Frostburn Canyon and make your way to the next marker. Note that it's above ground. If you're inside the caves, you won't find Zed's last group of abominations. Exit the caves (toward the icy-looking side) and find the marker location outside instead.

Shoot the monsters and turn in the quest in Sanctuary for Eridium and XP rewards.